Midnight in Helmand, and the only sound is the crump of boots on the desert soil. A line of Royal Marines prowls through the night, their rifles trained on the inky darkness. Suddenly a rattle of gunfire shatters the quiet. The commandos drop to the ground, train their guns in the direction of the noise, and wait.

Out there may have been a Taliban ambush or a drug lord whose high-speed convoys power through the night laden with opium and heroin. Moments later, Second Lieutenant Gordon Sweny sends an answer down the line. "ND," he whispers into his headset - negligent discharge. The gunfire, it seems, came from a nearby camp of the Afghan national army, Britain's key ally in Helmand.

Distinguishing friend from foe can be difficult in Helmand, the lawless Afghan province that will soon be home to one of Britain's most ambitious - and perilous - deployments to Afghanistan since colonial times.

By next May more than 3,300 British paratroopers, backed by Apache helicopters, Harrier warplanes and a phalanx of hi-tech artillery, will start pouring in. Their mission is to impose order and facilitate development in a lost province where violence, crime and bitter tribal rivalries are part of everyday life.

Helmand has concentrated doses of Afghanistan's most worrying problems: a corrupt local government and police; vast swaths of territory under the control of the Taliban; and a fast-growing drug industry. Last year Helmand produced more poppies, the plant used to make heroin, than any other Afghan province. This year the crop is expected to double.

"Nobody thinks this is going to be an easy ride," said Nick Kay, the newly appointed Foreign Office coordinator for southern Afghanistan.

Most of the British soldiers will be based in Camp Bastion, a sprawling base just off the province's main highway. A company of marines from 42 Commando unit arrived last week to protect it until it is completed in the early summer.

A US-funded £50m camp for the Afghan national army is under construction next door, which will house soldiers from the fledgling national force to be trained by British officers.

The British are keen to stress a difference in style from the departing American contingent, which is due to leave in mid-April. Whereas US soldiers roar through Laskhar Gah inside armoured vehicles, the British have started daily foot patrols in an effort to gain people's confidence.

On Thursday Drummer Philip Grundy, 21, balanced his S-80 rifle as he kicked a ball with a group of children. "Pashto? Ah, me no Pashto," he said, smiling over a cacophony of greetings.

But many Helmandis say it is security not smiles they want. The token international presence until now means that outside the two main towns, Laskhar Gah and Gereshk, the Taliban and drug lords hold sway. The militants terrorise teachers, aid workers or anyone linked to the central government. Once darkness falls the Taliban rules. In the latest attack four Afghan soldiers were gunned down in an ambush on Thursday night.

Many disillusioned Helmandis doubt the British are serious, said Sardar Muhammad of the Mercy Corps aid agency. "They think this is a change in name, nothing more," he said.

One of the British soldiers' first tasks will be to reclaim the night, said Colonel Henry Worsley, the commander in Laskhar Gar, the provincial capital. Although the Nato mandate does not allow British troops to aggressively seek fights with the Taliban, they do expect trouble. "I think it's common sense the enemy will have a go," he said.

The terrain is among the most challenging anywhere. Some mountain villages in northern Helmand are accessible only by donkey; the burning deserts of the south will test the hardiest vehicles.

British troops will avoid "busting down doors" or other search techniques used by US soldiers that have caused anger in the conservative south. "That's their way but it's not ours," said Col Worsley.

Officials stress the importance of strengthening President Hamid Karzai's government, which has only a tenuous toehold in Helmand. "There's got to be an Afghan face on this," said Col Worsley.

Britain is throwing its political weight behind the new governor, Muhammad Daud, who has promised to start eradicating the poppy crop shortly. But the campaign, if it goes ahead, is likely to prove unpopular and violent, and British troops will not be involved, said Col Worsley. "You won't see us turning up at some poor farmer's house, arresting him and chopping down his crops."

Instead British paratroopers and Apache helicopters will conduct week-long missions at Baramcha, a border town filled with drug smugglers and Taliban insurgents slipping across from neighbouring Pakistan. All operations will rely heavily on the fledgling Afghan forces, principally the national army. "They are our exit strategy - a well trained, well led Afghan army," said Col Worsley.

But, as the accidental firing incident during this week's night patrol showed, it may take British trainers some time to turn the army into a western-style fighting force, with a sense of national pride as well as fighting skills.

Among the hundreds of recruits at a military parade last week stood Hassan Gul, 25, who happily admitted that he had previously fought under the Taliban. "I like to fight for everyone," he said with a smile. "Whichever government comes along, I will serve with it."